The invention generally relates to the field of fusing powder toner images on receiver sheets.
Fusers for fusing powder toner images on receiver sheets, for examples images formed by an electrographic process, typically include a fuser roller and a pressure roller. In general, the fuser roller is heated, and the pressure roller is not heated. The pressure roller is subject to contamination, for example, from toner and paper residue. Most pressure roller contamination is cleaned by subsequent prints, as they pass through the fusing nip. Some pressure roller contamination however, remains on the pressure roller after a print run is completed. Pressure roller contamination that remains on the pressure roller after a print run can cause image defects, or even jams on subsequent print runs if the contamination is sufficiently severe.
It has thus been noted that both image defects and jams occur with a higher frequency just after a machine standby period. Heating of the fuser roller typically continues during standby. In the case of an externally heated fuser, heating of the fuser roller is accomplished through rotating contact with the heater rollers and pressure roller. This rotating contact also heats the pressure roller. Contamination is more easily removed from a hot pressure roller, and tends to be removed in an uncontrolled and incidental manner leading to image defects and jams.
A primary source of pressure roller contamination is cold offsetting from the backside of a print onto the pressure roller. Offset increases as run length increases since cold offset tends to increase as temperature is decreased, and pressure rollers are typically unheated during a run. Heating the pressure roller will reduce pressure roller contamination, but is expensive and may cause side effects such as print “bricking” and excessive curl since the prints receive more heat.
Since pressure roller contamination accumulates during continuous runs, and is inadvertently removed after standby periods, the higher the ratio of run length to standby, the higher the level of pressure roller contamination. Thus, as print rate and run length increase, the probability of pressure roller contamination also increases.